Exosomes are nano‐sized membrane vesicles which are released extracellularly after fusion of multivesicular endosomes with the cell membrane. Despite their characteristic composition of proteins compared to the cell membrane, no exosome‐specific molecule has so far been characterized. Exosomes are found in bronchoalveolar lavage (BAL), urine, serum and breast milk, and are released from several cells implicated in allergy including mast cells, dendritic cells (DC), T cells and epithelial cells. Antigen‐loaded exosomes have been shown to be highly immunogenic and we propose that exosomes could be a modulating factor in allergic responses. Allergen‐presenting exosomes could transport allergen and stimulate allergen‐specific T cells, and possibly also biasing T cell responses depending on the molecules present on the exosome surface. Furthermore, exosomes from mast cells, highly active in allergic reactions, have been found to induce DC maturation and also to be able to transport functional RNA to recipient cells, suggesting a new pathway for cell communication. Reversely, tolerizing exosomes e.g. tolerosomes, from gut or breast milk, could block an allergic response or prevent allergy development. A better understanding of the role of exosomes in allergies could make us understand how allergy can be prevented or lead to the development of more efficient treatments.

SummaryOral tolerance is an active process that starts with sampling of luminal antigens by the intestinal epithelial cells (IEC), followed by processing and assembly with major histocompatibility complex class II and subsequently a release of tolerogenic exosomes (tolerosomes) from the IEC. We have previously shown that tolerosomes can be isolated from serum shortly after an antigen feed, and will potently transfer antigen-specific tolerance to naive recipients. Here we study the capacity of the tolerosomes to protect against allergic sensitization in a mouse model of allergic asthma. Serum or isolated serum exosomes from tolerized BALB/c donor mice were transferred to syngeneic recipients followed by sensitization and intranasal exposure to ovalbumin (OVA). Blood, bronchoalveolar lavage (BAL) and lymph nodes were sampled 24 hr after the final exposure. The number of eosinophils was counted in BAL fluid and the levels of immunoglobulin E (IgE) and OVA-specific IgE were measured in serum. Mediastinal and coeliac lymph nodes were analysed by flow cytometry. The animals receiving serum from OVA-fed mice displayed significantly lower numbers of airway eosinophils and lower serum levels of total IgE as well as of OVA-specific IgE compared with controls. Moreover, the tolerant animals showed a significantly higher frequency of activated T cells with a regulatory phenotype in both mediastinal and coeliac lymph nodes. The results show that serum or isolated serum exosomes obtained from OVA-fed mice and administered intraperitoneally to naive recipient mice abrogated allergic sensitization in the recipients.

a b s t r a c t B lymphocytes are essential antibody-producing cells of the immune system. During the development of progenitor B cells to mature B cells that express a membrane-bound antibody, the B cell receptor (BCR), the cells undergo selection at several checkpoints, which ensures that a diverse antibody repertoire is generated and that the BCRs recognise foreign-, but not self-, antigens. In this review, we consider the pre-BCR checkpoint. Mutations or alterations that affect this checkpoint underpin the development of pre-B cell leukemias, primary immunodeficiency, and possibly, systemic autoimmunity. Ó 2010 Federation of European Biochemical Societies. Published by Elsevier B.V. All rights reserved. The pre-B cell receptorFollowing the discovery of pre-B cells as precursors of B cells, the pre-B cell receptor (pre-BCR) was identified as a 'precursor' of the B cell receptor (BCR) [1][2][3]. The observation that the leukemic cells in most children with acute lymphoblastic leukemia were classified as pre-B cells [4], suggested the importance of this developmental stage. Whereas the BCR is assembled from the immunoglobulin (Ig) heavy (H) and light (L) chains, the pre-BCR is assembled from Ig H and surrogate L (SL) chains, the latter composed of the invariant k5 and VpreB polypeptides and having homology to bona fide Ig L chains [5][6][7][8]. Since the discovery of the pre-BCR, several groups have investigated the roles of the pre-BCR and SL chain [9]. It is clear that the pre-BCR is essential for B cells to develop normally, and that mutations or alterations affecting the pre-BCR underpins the development of cancer and immunodeficiency, and perhaps even autoimmunity [3,10,11]. Antibody diversityAntibodies are mediators of humoral immune responses, which participate in protection against pathogens. An antibody consists of an identical pair of H and L chains, each with a variable and a constant region, which are responsible for antigen recognition and effector functions, respectively (Fig. 1). Within the variable region, three hypervariable regions (known as 'complementarity determining regions'; CDR1-3) determine antigen recognition. This hypervariability is established at the DNA level through a somatic process termed V(D)J recombination, whereby variable (V), diversity (D), and joining (J) gene segments are recombined [12]. The mouse H chain locus, for instance, contains >100 functional V gene segments, >10 D segments, and four J gene segments [13]. Together with recombinations of the L chain loci, this diversity theoretically yields >10 12 different antibodies.The products of the lymphoid-specific recombination-activating genes (RAG1 and RAG2) are required for the recombination process [14,15]. These proteins bind the recombination signal sequences that flank each gene segment, and this is followed by a doublestrand break in the DNA, which is the initial event in the bringing together of the gene segments (Fig. 2).The diversity results from the V H gene segment itself (which encodes CDR1 and CDR2), the rec...

No abstract

The hygiene hypothesis suggests that lack of microbial stimulation in early infancy may lead to allergy, but it has been difficult to identify particular protective microbial exposures. We have observed that infants colonised in the first week(s) of life with Staphylococcus aureus have lower risk of developing food allergy. As many S. aureus strains produce superantigens with T-cell stimulating properties, we here investigate whether neonatal mucosal exposure to superantigen could influence the capacity to develop oral tolerance and reduce sensitisation and allergy. BALB/c mice were exposed to staphylococcal enterotoxin A (SEA) as neonates and fed with OVA as adults, prior to sensitisation and i.n. OVA challenge. Our results show that SEA pre-treated mice are more efficiently tolerised by OVA feeding, as shown by lower lung-cell infiltration and antigen-specific IgE response in the SEA pre-treated mice, compared with sham-treated mice. This was not due to deletion or anergy of lymphocytes by SEA treatment, because the SEA pre-treated mice that were fed with PBS showed similar inflammatory response as the sham-treated PBS-fed mice. Our results suggest that strong T-cell activation in infancy conditions the mucosal immune system and promotes development of oral tolerance.Key words: Allergic sensitisation . Mucosal immunity . Staphylococcal enterotoxin A . Tolerance Supporting Information available online IntroductionAllergies have increased markedly in Western industrialised countries, where they now afflict every third child. Allergies are linked to wealth, good education and small families, observations that have given rise to the hygiene hypothesis, according to which the developing immune system should be exposed to appropriate microbial stimulation in early childhood in order to mature correctly and for allergies to be avoided [1,2]. The hygiene hypothesis is also supported by experimental data. Germ-free animals are less prone to develop oral à These author contributed equally to this work. Eur. J. Immunol. 2009. 39: 447-456 DOI 10.1002 Immunomodulation 447 tolerance than animals reared conventionally [3,4]. Oral tolerance is induced by passage of antigens over the gut mucosa, prevents against both Th1-and Th2-dominated immune responses [5] and includes development of Treg [6,7]. Natural thymic-dependent Treg (CD4 1 CD25 hi FoxP3 1 ) are fundamental in protection against autoimmunity, gut inflammation and IgE responses. This cell subset has reduced functional capacity in germ-free animals [8].The strongest T-cell activators known are the ''superantigens'', exotoxins produced by certain pathogenic bacteria such as Staphylococcus aureus. These include staphylococcal enterotoxin A, B, C, D and E, as well as toxic shock syndrome toxin-1. The superantigen binds to the variable part of the TCR and to the MHC class II molecule, thereby ''mimicking'' antigen recognition, which leads to T-cell activation [9][10][11][12]. As many as 10-30% of all T cells can become activated by a certain superantigen compared with o0...